The present invention relates to a flexible flat circuit, such as an FPC (Flexible Printed Circuit) or an FFC (Flexible Flat Cable), having a plurality of conductors with different current capacities.
FIG. 6 is a view illustrating the flexible flat circuit disclosed in JP-A-2004-23978 described below.
In this flexible flat circuit 100, a plurality of conductors 111 and 112 for providing a circuit are formed on an insulation sheet 101 made of an insulating resin in a separated state. Furthermore, a fuse circuit 120 for making a connection between the two conductors 111 and 112 is provided on the conductors 111 and 112. An insulation sheet, not illustrated, for covering the conductors 111 and 112 and the fuse circuit 120 described above is formed in a laminated state on the insulation sheet 101. The insulation sheet for covering the conductors 111 and 112 and the fuse circuit 120 is identical with the insulation sheet 101.
In other words, the flexible flat circuit 100 is configured that the plurality of conductors 111 and 112 and the fuse circuit 120 are held between the pair of insulation sheets 101.
Conventionally, the conductors 111 and 112 on the insulation sheet 101 are formed so as to have a uniform thickness by a subtractive method or an additive method.
The subtractive method is a method for forming the conductors 111 and 112, wherein a copper foil is attached to the entire surface of the insulation sheet 101, a coating film layer serving as a corrosion resistant film is formed in regions to be left as the conductors 111 and 112 on the copper foil, and the copper foil on unnecessary portions (regions having no coating film layer) are eliminated by etching.
The active method is a method for forming the conductors 111 and 112, wherein a resist (plating resist) is formed on the portions of the surface of the insulation sheet 101 on which the conductors 111 and 112 are not desired to be formed, and electrolytic plating or electroless plating is performed on portions having no resist.
Furthermore, in the case of the conventional flexible flat circuit 100, the fuse circuit 120 is formed as an independent component equipped with a pair of connection terminal sections 121 to be connected to the conductors 111 and 112 by soldering or the like and a fusible body 122 for making a connection between the pair of connection terminal sections 121, and then soldered to the conductors 111 and 112. The material and the cross-sectional dimensions of the fusible body 122 are set so that the fusible body 122 is fused when a current equal to or more than the rated current flows between the pair of connection terminal sections 121.
However, in the case of the flexible flat circuit 100 according to JP-A-2004-23978, the conductors 111 and 112 for forming a circuit are formed by the subtractive method or the additive method and have a uniform thickness. Hence, in the case that the current capacities of the respective conductors 111 and 112 are changed depending on the rating of an electric circuit serving as the connection destination of the conductors 111 and 112, the conductors 111 and 112 are required to secure necessary cross-sectional areas by changing the width dimensions of the respective conductors 111 and 112.
In other words, for example, when a conductor which is connected to a power source (battery) and through which large currents flow is compared with a conductor through which small currents, such as control signals, flow, the width of the former conductor, through which the large currents flows, is required to be set large. The increase in the width of the conductor results in an increase in the width dimension of the flexible flat circuit 100 itself. This causes a problem of the upsizing of the flexible flat circuit 100.
In addition, in the case of the flexible flat circuit 100 according to JP-A-2004-23978, the fuse circuit 120 to be mounted on the flexible flat circuit 100 is formed as a separate component and then connected to the conductors 111 and 112 by soldering or the like. This causes problems, i.e., an increase in the number of components, an increase in the number of processing steps, an increase in cost, and a decrease in productivity.
Accordingly, for the purpose of solving the above-mentioned problems, an object of the present invention is to provide a flexible flat circuit on which conductors with large current capacities and having the same width dimension as that of conductors with small current capacities can be mounted, whereby the width dimension of the flexible flat circuit can be shortened. Furthermore, another object of the present invention is to provide a flexible flat circuit on which a fuse circuit can be mounted easily on a conductor formed on the flexible flat circuit.